1. Field of the Invention
The present invention relates to a transfer belt unit that includes a transfer belt and an image forming apparatus that includes the transfer belt unit.
2. Description of the Related Art
An electrophotographic image forming apparatus has been used in which a toner image formed on a photoconductor as an image carrier is primarily transferred onto an endless transfer belt, and the transferred toner image is carried by endless movement of the transfer belt, and secondarily transferred onto a recording medium.
In such an image forming apparatus, it is necessary to apply a tension to the transfer belt to endlessly move the transfer belt in a tensioned state, so that a toner image primarily transferred onto the transfer belt can be secondarily transferred onto the recording medium accurately. However, if the tension is applied at all times to the transfer belt, deformation of a fixing belt may occur in a portion with a small curvature factor when the apparatus stops. The deformation is referred to as curling.
Japanese Patent Application Laid-open No. S61-130972 discloses an apparatus including a mechanism that applies an appropriate tension to the transfer belt at the time of driving the apparatus, and loosening the tension applied to the transfer belt at the time of stopping the apparatus. By loosening the tension at the time of stopping the apparatus, it is possible to prevent curling that occurs in a portion with a small curvature factor when the apparatus stops.
However, a mechanism that adjusts the tension automatically is required to apply the appropriate tension at the time of driving the apparatus, and loosening the tension at the time of stopping the apparatus, which makes the apparatus complicated, and causes a cost increase.
Through extensive researches by the inventor of the present invention, it has been found that curling likely occurs as a tensile elastic stress of material of the transfer belt increases, and the occurrence of curling can be prevented by using a material having a lower tensile elastic stress without a mechanism for adjusting the tension.
If, however, a material having a lower tensile elastic stress is used for the transfer belt, cracking at the end of the belt or belt waving may occur depending on the settings. The cracking at the end of the belt means that an end of the transfer belt cracks because the end in the width direction of the transfer belt comes in contact with a regulating member that regulates the movement in an axial direction of the belt at an axial end of at least one of a plurality of stretching rollers. The belt waving means that the end of the belt comes in contact with the regulating member and deforms, which causes the transfer belt to become wavy on the downstream side from the contact portion in an endless moving direction.